Roadside transmitter

ABSTRACT

In a communication system between a road and a vehicle, communicating between a plurality of road communication stations arranged in a cell and a vehicle-mounted mobile station within the cell, a vehicle-mounted transmitting equipment, in which communication is not interrupted even when a received frequency varies due to the Doppler effect and which performs stable communication, is realized. A transmitting antenna  36   a  having a directivity in the running direction of the vehicle and a transmitting antenna  36   b  having a directivity opposite to the running direction of the vehicle are arranged. An offset for increasing a frequency is provided for a radio wave emitted from the antenna  36   a  while an offset for lowering a frequency is provided for a radio wave emitted from the antenna  36   b.  The variation in the received frequency depending on the Doppler shift is reduced for the vehicle-mounted receiving device to facilitate automatic frequency control (AFC), thereby reducing degradation in the quality of data after demodulation.

TECHNICAL FIELD

The present invention relates to a road transmission equipment used in acommunication system between a road and a vehicle, allowing mobilecommunication between a road and a mobile station by locating aplurality of road antennas along the road to form a cell on the road.

BACKGROUND ART

There is an increasing demand for communications between roadcontrollers and vehicles. On a superhighway, in particular, to enable avehicle to operate on the road without any burden on the driver and anyaccident both for the controller and the driver, frequent interchange ofinformation is necessary between the road and the vehicle. One type ofsuch a developed system is a self-operating system that allows a vehicleto run with close communication between the road and the vehicle, whichare equipped with various sensors and a camera (see, for example,Japanese Unexamined Patent Publication No. 241495 of 1996).

For the construction of a driving support system (hereinafter, referredto as “communication system between a road and a vehicle”) which makesuse of the communication with the vehicle for future extension into aself-operating system, it is necessary to provide a communication area(cell) on the road.

To provide such a cell, we may consider laying a leakage coaxial cablealong the road. However, the drawback of this method is that large-scaleconstruction is needed for laying such a cable. In addition, since it isrequired to locate the leakage coaxial cable at a relatively lowposition on the ground, the distance for which a radio wave propagatesin a direction across a traffic lane is disadvantageously short.

On the other hand, if the communication is performed with a plurality ofroad antennas being arranged on the road at predetermined intervals, asingle road antenna can cover a relatively large cell. In this case,each of the road antennas is connected to a central base station of theroad controller via an optical fiber and the like.

In the case where the road antennas are provided, when a large-sizevehicle comes proximate to a small-size vehicle, it obstructs the viewof the driver of the small-size vehicle, preventing him from seeing theroad antenna from inside the small-size vehicle. In particular, it islikely that a microwave or a millimeter wave of a high frequency havinga small angle of diffraction is blocked. Accordingly, the communicationbetween the vehicle and the road is interrupted, thereby preventingcontinued communications.

Therefore, in order to enable continuous communications between the roadand the vehicle, multi-station communication has been proposed Accordingto this multi-station communication, a plurality of road antennas havingan inherent directivity are provided along the road, and radio waves ofthe same frequency and the same content are emitted from the respectiveroad antennas toward the same cell.

A multi-station communication system is advantageous because such asystem has a plurality of propagation paths for radio waves to beemitted and therefore the radio wave avoids being blocked so as tocontinuously perform smooth communication between a mobile station and aroad communication station even when a vehicle runs proximate to alarge-size vehicle such as a truck.

In the multi-station communication system, however, the Doppler effectoccurs when a vehicle moves. The antennas receiving radio waves from thefront and behind receive radio waves of respectively differentfrequencies based on Doppler shift.

FIG. 9(a) shows an arrangement of conventional road antennas a, b, and cin a multi-station communication system and a vehicle running underthese antennas. A receiving antenna 61 and a receiving device 4 aremounted on the vehicle.

FIG. 9(b) is a graph showing the transitions of deviations of thefrequencies received by the receiving antenna 61. The transition of adeviation of the frequency received by the receiving antenna 61 from theroad antenna a is indicated by a line a; the transition of a deviationof the frequency received by the receiving antenna 61 from the roadantenna b is indicated by a line b, and the transition of a deviation ofthe frequency received by the receiving antenna 61 from the road antennac is indicated by a line c.

An exemplar value of the deviation of the frequency received from theroad antenna a will be given. Doppler shift Δf is expressed by: Δf=f0v/c (c is a velocity of light), where a transmitted frequency of theroad antenna is f0, and v is a velocity of a vehicle. When the vehicleruns on the road, the Doppler shift Δf is given by:

Δf=f 0(v/c)L(L ² +H ²)^(−½)

where the height of the road antenna from the ground is H, and adistance between the vehicle and the road antenna is L. Assuming f0=5.8GHz, v=100 km/h, and H=10 (m), the Doppler shift Δf is expressed by:

Δf=537·L(L ² +H ²)^(−½)(Hz)

When the distance between the road antennas is set to 50 (m), the valueof L ranges from 0 (m) to 50 (m). Accordingly, the Doppler shift Δfranges from 0 to 527 (Hz). At the middle point between the antennas,i.e. L=25 (m), the Doppler shift Δf is 499 (Hz).

With the arrangement of the road antennas as shown in FIG. 9(a), thereoccurs skipping of received frequencies as shown in FIG. 9(b) each timea vehicle passes near the middle point between the antennas. Thisskipping is caused because the automatic frequency control (AFC) of thereceiving device 4 is drawn toward the frequency having greaterreceiving power. This skipping makes it difficult to follow thefrequency control in a receiving section, resulting in the interruptionof communication during the occurrence of skipping.

Accordingly, it is desired to reduce the Doppler effect on the side ofthe road-transmitting device in the communication system between a roadand a vehicle performing communication between a plurality of roadcommunication stations arranged in a cell and a vehicle-mounted mobilestation within the cell.

DISCLOSURE OF THE INVENTION

(1) A road transmission equipment as set forth in claim 1 with the viewof achieving the abovementioned object, comprises:

a first transmitting antenna having a directivity in a running directionof a vehicle,

a second transmitting antenna having a directivity in a directionopposite to the running direction of the vehicle, a first transmittingsection and a second transmitting section respectively connected to thefirst transmitting antenna and the second transmitting antenna to outputsignals of the same frequency, and

a frequency correction section, wherein;

the frequency correction section performs correction so as to providethe first transmitting section with a positive frequency offset forincreasing a frequency of a signal supplied to the first transmittingantenna, and so as to provide the second transmitting section with anegative frequency offset for lowering a frequency of a signal suppliedto the second transmitting antenna.

In the present invention, an offset for increasing the frequency isprovided for an radio wave directed in the running direction of thevehicle while an offset for lowering the frequency is provided for anradio wave directed in the opposite direction to the running directionof the vehicle, for transmission of these radio waves.

Therefore, the variation in the received frequency based on the Dopplershift is reduced for the vehicle-mounted receiving device to lessen therequirements for frequency control of automatic frequency control (AFC).Thus, the degradation of the quality of data after demodulation isreduced.

(2) It is preferred that the amounts of the positive frequency offsetand the negative frequency offset provided by the frequency correctionsection are equal to each other (claim 2).

The reason being that since the running speed of the vehicle is normallyalmost consistent within the cell, the amount of Doppler shift of theradio wave directed in the running direction of the vehicle, to whichthe vehicle-mounted receiving device is subjected, is also considered tobe the same as that of the radio wave directed in the opposite directionto the running direction, to which the vehicle-mounted receiving deviceis subjected.

(3) The road transmission equipment according to the present inventionmay further comprise a speed detection means for detecting the speed ofthe vehicle running in the cell, wherein

the frequency correction section may set the amount of the frequencyoffset based on the detected speed of the vehicle (claim 3).

Since the amount of Doppler shift of the vehicle-mounted receivingdevice can be obtained if the running speed of the vehicle can bedetected, the amount of a frequency offset can be set based on theamount of the Doppler shift. Accordingly, in the case where the speed ofthe vehicle changes with time, accurate frequency correction can beperformed in real time.

When a plurality of vehicles are present in the cell and the speed ofeach vehicle can be detected, the amount of frequency offset is setbased on the average value of the speeds of a plurality of vehicles.

(4) The amount of the frequency offset provided by the frequencycorrection section may be set to a fixed value on the assumption thatthe vehicle is subjected to constant Doppler shift (claim 4).

Normally, it is considered that the running speed of a vehicle is almostalways consistent within the same cell on the same road and does notgreatly change with time (although the running speed changesconsiderably in the case of traffic restriction or traffic congestion,the frequency and the duration of traffic restriction or trafficcongestion cannot be predicted).

Therefore, even with the fixed amount of frequency offset, the object ofthe present invention of reducing the variation in the receivedfrequency based on the Doppler shift can be achieved.

Moreover, since the speed detection means is no longer needed, theconfiguration of the road transmission equipment is advantageouslysimplified.

(5) The first transmitting section and the second transmitting sectionmay transmit an orthogonal frequency division multiplex (OFDM) modulatedradio wave (claim 5).

In a case where an OFDM modulation method is used for dividingtransmitted information into subcarriers and transmitting the obtainedsubcarriers, the sensitivity of a bit error rate with respect to afrequency disarrangement is high because a distance between thefrequencies of adjacent subcarriers is small. Accordingly, in aconventional communication system between a road and a vehicle asillustrated in FIG. 9, a Doppler frequency change increases, therebydegrading the transmission characteristics.

Since the correction for providing a frequency with an offset isperformed so as to reduce a Doppler frequency change in the presentinvention, the present invention is extremely effective for acommunication system between a road and a vehicle using an OFDMmodulation method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view showing the configuration of a communicationsystem between a road and a vehicle.

FIG. 2 is a block diagram showing the internal configuration of atransmitting device 2 b of a transmitting and receiving station 2.

FIG. 3 is a graph illustrating symbol transmission by OFDM on afrequency axis f and a time axis t.

FIG. 4 is a block diagram showing an exemplary modification of theinternal configuration of the transmitting device 2 b shown in FIG. 2.

FIG. 5 is a circuit diagram showing an f_(d) correction circuit 37 ofthe transmitting device 2 b shown in FIG. 4.

FIG. 6 is a block diagram showing the internal configuration of areceiving device 2 a of the transmitting and receiving station 2.

FIG. 7 is a conceptual view showing the configuration of avehicle-mounted mobile station 4.

FIG. 8(a) is a layout drawing showing the arrangement of road antennasa1, a2; b1, b2; c1 and c2 of the road transmission equipment accordingto the present invention and a vehicle running thereunder; and FIG. 8(b)is a graph showing the transition of a deviation Δf of a frequencyreceived by the vehicle-mounted mobile station.

FIG. 9(a) is a layout drawing showing the arrangement of three roadantennas a, b, and c of a conventional multi-station communicationsystem and a vehicle running thereunder; and FIG. 9(b) is a graphshowing the transition of a deviation of a received frequency.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

Hereinafter, a preferred embodiment of the present invention will bedescribed in detail with reference to the attached drawings.

FIG. 1 is a conceptual view showing the configuration of a communicationsystem between a road and a vehicle. This communication system between aroad and a vehicle transmits and receives road transportationinformation between a road communication station and a mobile stationmounted on a vehicle.

A cell is formed along the road. In the cell or in the vicinity thereof,a plurality of transmitting and receiving stations 2 are arranged atintervals. Each of the transmitting and receiving stations 2 has anantenna 36 a having a forward directivity and an antenna 36 b having arearward directivity along a running direction of the vehicle.

A radio wave having a frequency offset toward the positive side isemitted into the cell from the antennas 36 a having a forwarddirectivity, while a radio wave having a frequency offset toward thenegative side is emitted into the cell from the antennas 36 b having arearward directivity.

The radio waves emitted from the antennas have the same frequency withexclusion of the offsets.

The transmitting and receiving station 2 acquires transmitted data via awire transmission line 9 such as an optical fiber or a coaxial cable(although a wireless transmission line may be used instead of the wiretransmission line, it is assumed hereinafter to use the “wiretransmission line 9”) from a central base station 1. The transmitteddata is then subjected to OFDM modulation using a plurality of carrierwaves (subcarriers) orthogonally crossing each other, and is transmittedas a wireless radio wave into the cell. The transmitting and receivingstation 2 receives the OFDM modulated wireless radio wave from thevehicle-mounted mobile station 4 in the cell, and then perform OFDMdemodulation on this wireless radio wave so as to transmit the receiveddata via the wire transmission line 9 from the central base station 1.

The combination of the function of the transmitting and receivingstation 2 and the function of the central base station 1 will bereferred to as the “road communication station”.

The reason for use of the OFDM modulation method is as follows.

In the case of multi-station communication, since a plurality of radiowaves are emitted with similar transmission power into the same cell,phasing is caused by multipath. As a result, intercarrier interferenceor intersymbol interference frequently occurs. Consequently, it isessential to remove the effects of such interference in the systemconstruction.

Generally, a mobile object communication system using a single carrieris likely to be subjected to the effect of intersymbol interference bythe multipath delayed wave.

Accordingly, it is proposed to use the OFDM modulation method that iscapable of transmitting a plurality of subcarriers obtained by dividinga carrier. The OFDM modulation method is advantageous in that theeffects of a delayed wave can be eliminated by setting a guard time.

FIG. 2 is a block diagram showing the internal configuration of atransmitting device 2 b of the transmitting and receiving station 2.

The transmitting device 2 b comprises a serial/parallel (S/P) convertingcircuit 31, an fd setting circuit 32, an inverse Fourier transformcircuit 33, QPSK modulation circuits 34 a and 34 b, up-converters 35 aand 35 b and the like.

The inverse Fourier transform circuit 33 realizes various functions asfollows. The inverse Fourier transform circuit 33 performs inverseFourier transform on the transmitted data supplied in parallel from theS/P converting circuit 31, converts the inverse Fourier transformed datato return it to serial, and time-compresses a serial symbol string so asto move a posterior symbol to the beginning of the string, therebysetting a guard time.

FIG. 3 is a graph showing the symbol transmission by OFDM on a frequencyaxis f and a time axis t. An effective symbol length is represented byTS, and a guard time is given by Δt. A time compression ratio isrepresented by (TS+Δt)/TS. In the case of QPSK, TS is expressed by:

TS=2 n/m(μsec)

where the number of subcarriers is n, and the transmission rate is m(Mbps).

It is required to set the guard time Δt of the OFDM modulation to belonger than the time delayed by the multipath. In this way, thetransmitting and receiving station 2 and the vehicle-mounted mobilestation 4 can avoid intersymbol interference so as to accurately restorethe received signal without being adversely affected by the propagationdelay due to the presence of a plurality of propagation paths for theradio wave (multipath).

With reference to FIG. 2, the QPSK modulation circuits 34 a and 34 bperform QPSK transform by D/A converting a signal corresponding to thephase 0° and a signal corresponding to the phase 180°, and a signalcorresponding to the phase 90° and a signal corresponding to the phase270° which are output from the inverse Fourier transform circuit 33,subjecting these signals respectively to a sin wave and a cos wave, andadding them.

Needless to say, although QPSK modulation is performed in thisembodiment, other modulation methods, for example, QAM, BPSK, 8PSK andthe like may be used. In the following description, however, it isassumed that QPSK modulation is performed unless specifically noted.

The up-converters 35 a and 35 b are circuits for frequency conversioninto a wireless frequency. The output signals from the up-converters 35a and 35 b pass through a circulator and a coaxial cable to be emittedfrom road antennas 36 a and 36 b as radio waves.

Herein, a method for providing a frequency with an offset in the QPSKmodulation circuit 34 will be described.

The f_(d) setting circuit 32 is a circuit for setting an offsetfrequency f_(d). For this setting of the offset frequency, there aremethods of: (1) detecting running speed of a vehicle moving on the roadin real time, thereby setting the offset frequency; and (2) previouslygiving the offset frequency as a constant.

For detecting the running speed of a vehicle according to the method(1), for example, there are (1-1) a method for obtaining average runningspeed of the vehicles based on the speed information transmitted fromeach vehicle within the cell; (1-2) a method for obtaining averagerunning speed by detecting the speed of each vehicle with an ultrasonicspeed sensor or a television camera being arranged on the road; and(1-3) a method for obtaining the average running speed of the vehiclesby detecting the speed of each vehicle based on Doppler shift Δfdetected on automatic frequency control (AFC) in the receiving device 2a.

In the method (2) of previously giving an offset frequency as aconstant, it is assumed that the vehicle always runs at the speed whichhas been previously obtained in a statistical or experimental fashionwithin the cell. On the road where traffic is constant, theconfiguration can be simplified with the running speed of a vehiclebeing fixed as a constant rather than with the running speed detected inreal time because it is not necessary to provide a running speeddetection means.

The amount of the offset frequency fd is set to be a half of the maximumDoppler shift Fd (fd=Fd/2) which a vehicle experiences in a case wherethe vehicle runs at said speed.

A signal corresponding to the thus set offset frequency+fd is suppliedto a voltage control oscillation (VCO) circuit. Then, a signal having anangular frequency of ω+2πfd is generated by a PLL oscillator. Afterbeing provided with a phase difference of 90° by a phase-shift circuit,the signal having an angular frequency of ω+2πfd is supplied to the QPSKmodulation circuit 34 a.

On the other hand, a signal corresponding to the offset frequency−fd issupplied to a VCO circuit. Then, a signal having an angular frequency ofω−2πfd is generated by the PLL oscillator. After being provided with aphase difference of 90° by the phase-shift circuit, the signal having anangular frequency of ω−2πfd is supplied to the QPSK modulation circuit34 b.

As a result, a frequency signal having an offset of+fd is obtained fromthe QPSK modulation circuit 34 a while a frequency signal having anoffset of−fd is obtained from the QPSK modulation circuit 34 b.

FIG. 4 is a block diagram showing an exemplary modification of theinternal configuration of the transmitting device 2 b shown in FIG. 2.

In comparison, the circuit configuration shown in FIG. 4 differs fromthat shown in FIG. 2 in that a signal I having an in-phase component anda signal Q having an orthogonal component to be input into the QPSKmodulation circuit 34 are subjected to frequency correction so as to beprovided with offsets while signals having angular frequencies of ω±2πf_(d) are supplied to a local oscillation circuit of the QPSKmodulation circuit 34 so as to be provided with offsets in the circuitconfiguration of FIG. 2.

A circuit for correcting the offset frequency is an f_(d) correctioncircuit 37.

FIG. 5 is a circuit diagram showing the internal configuration of the fdcorrection circuit 37. The fd correction circuit 37 produces signals ofcos(2πfdt) and sin(2πfdt) based on the information of the offsetfrequency fd obtained from the fd setting circuit 32. Then, the signal Ihaving an in-phase component and the signal Q having an orthogonalcomponent output from the inverse Fourier transform circuit 33 arerespectively multiplied by cos(2πfdt) and sin(2πfdt) to obtain foursignals:

I cos(2πf_(d)t),

I sin(2πf_(d)t),

Q cos(2πf_(d)t), and

Q sin(2πf_(d)t)

Furthermore, these four signals are added and subtracted so as to obtainIa, Ib, Qa and Qb:

Ia=I cos(2πf _(d) t)−Q sin(2πf _(d) t)

Qa=Q cos(2πf _(d) t)+I sin(2πf _(d) t)

 Ib=I cos(2πf _(d) t)+Q sin(2πf _(d) t)

Qb=Q cos(2πf _(d) t)−I sin(2πf _(d) t)

Then, Ia and Qa are supplied to the QPSK modulation circuit 34 a whileIb and Qb are supplied to the QPSK modulation circuit 34 b.

As a result, a frequency signal having an offset of +f_(d) is obtainedfrom the QPSK modulation circuit 34 a while a frequency signal having anoffset of−f_(d) is obtained from the QPSK modulation circuit 34 b.

FIG. 6 is a block diagram showing the internal configuration of thereceiving device 2 a of the receiving and transmitting station 2.

The receiving device 2 a comprises a receiving antenna 21, adown-converter 22, a QPSK demodulation circuit 23, a Fourier transformcircuit 24, a P/S (parallel/serial) converting circuit 26, a Δfdetecting section 27, and the like.

The down-converter 22 of the receiving device 2 a is a circuit whichconverts a wireless frequency into an intermediate frequency.

In contrast with the QPSK modulation circuit 34, the QPSK demodulationcircuit 23 performs QPSK demodulation, wherein one of two dividedsignals is subjected to a sin wave while the other divided signal issubjected to a cos wave whose phase differs by 90° from that of the sinwave so as to A/D convert these divided signals.

The frequency difference Δf detecting section 27 detects a deviation Δfof the received frequency based on the in-phase component I (signalafter being subjected to a cos wave) and the orthogonal component Q(signal after being subjected to a sin wave) of the QPSK demodulationcircuit 23. The deviation Δf of the received frequency can be obtainedbased on a difference between a deflection angle (I/Q)_(t) of a currentI/Q and a deflection angle (I/Q)_(t−1) sampled immediately before I/Qwhich are obtained by calculating the deflection angle of a complexnumber I/Q at sampling time intervals. Δf=(I/Q)_(t)−(I/Q)_(t−1)

The Δf detecting section 27 feeds back the deviation Δf of the receivedfrequency to the down-converter 22 and the QPSK demodulation circuit 23,thereby accomplishing the function of correcting the deviation Δf of thereceived frequency.

The Fourier transform circuit 24 performs processing that is opposite tothat of the inverse Fourier transform circuit 33 on the transmissionside. The Fourier transform circuit 24 performs Fourier transform on theQPSK demodulated signal with the effective symbol length TS as a windowlength, thereby obtaining a demodulated signal.

The P/S converting circuit 26 converts a Fourier transformed parallelsignal into a serial signal.

This data converted into a serial signal is transmitted to the centralbase station 1.

Next, the configuration of a vehicle-mounted mobile station to bemounted in the vehicle will be described.

FIG. 7 is a conceptual view showing the configuration of avehicle-mounted mobile station 4. The vehicle-mounted mobile station 4consists of a transmitting and receiving antenna 61, a receivingsection, a transmitting section and a frequency control section.

The transmitting section comprises a S/P converting circuit 47, aninverse Fourier transform circuit 49, a QPSK modulation circuit 50, andan up-converter 51 and the like.

The description for actions of the transmitting section is hereinomitted because the configuration of the transmitting section iswell-known and is identical with the main part of the configuration ofthe transmitting device 2 b on the road shown in FIG. 2.

The receiving section comprises a down-converter 66 for converting awireless frequency into an intermediate frequency, a QPSK demodulationcircuit 63, a Fourier transform circuit 64, a P/S converting circuit 65and the like. Since the configuration of the receiving part is alsowell-known and is similar to that of the receiving device 2 a describedwith reference to FIG. 6, the description thereof is herein omitted.

The frequency control section has the function of detecting a deviationΔf of the received frequency of the receiving section and the functionof performing frequency control of the receiving section based on thedeviation Δf

The function of detecting the deviation Δf of the received frequency canbe described in the same manner as that for the function of the Δfdetecting section 27 described with reference to FIG. 6. Specifically,deflection angles I/Q of a complex number I/Q are calculated at sampletime intervals based on the in-phase component I (signal after beingsubjected to a cos wave) and the orthogonal component Q (signal afterbeing subjected to a sin wave) of the QPSK demodulation circuit 63. Thedeviation Δf of the received frequency is then detected based on thedifference between the deflection angle (I/Q)_(t) of the current I/Q anda deflection angle (I/Q)_(t−1) sampled immediately before.

Δf=(I/Q)_(t)−(I/Q)_(t−1)

The frequency control section feeds back the detected deviation Δf ofthe received frequency to an oscillator of the down-converter 66,thereby accomplishing the function of correcting the deviation Δf of thereceived frequency.

f=d _(org) −Δf

where f_(org) is a frequency at which oscillation occurs when Δf is 0.

FIG. 8 is a graph showing the thus obtained deviations Δf offrequencies.

FIG. 8(a) shows the arrangement of road antennas a1, a2; b1, b2; c1 andc2 of a road transmission equipment of the present invention and avehicle running thereunder. On the vehicle, as described above, atransmitting and receiving antenna (hereinafter, simply referred to as a“receiving antenna”) 61 and a vehicle-mounted mobile station 4 aremounted.

FIG. 8(b) is a graph showing the transitions of the deviations Δf offrequencies. The transition of a deviation of the frequency that thereceiving antenna 61 receives from the road antenna a2 is indicated bythe line a2, the transition of a deviation of the frequency that thereceiving antenna 61 receives from the road antenna b1 is indicated bythe line b1, the transition of a deviation of the frequency that thereceiving antenna 61 receives from the road antenna b2 is indicated bythe line b2, and the transition of a deviation of the frequency that thereceiving, antenna 61 receives from the road antenna c1 is indicated bythe line c1.

As can be seen in the graph of FIG. 8(b), the deviation Δf of thefrequency is reduced to be almost half of that in FIG. 9 owing to thefrequency offset fd of the transmitting device 2 b on the road. Forinstance, taking b1 as an example, if there is no frequency offset fd,the vehicle is subjected to the Doppler shift of Fd at the maximum.However, since the frequency offset fd has been previously provided, theDoppler shift to which the vehicle is subjected is halved immediatelyunder the road antennas al and a2.

Moreover, it is understood that the skipping of a frequency occurring inthe vicinity A of the middle position between road antennas withmovement of the vehicle is almost halved. Accordingly, in the case whereautomatic frequency control (AFC) is performed, the followability tofrequency control is sufficient. As a result, the communication is notinterrupted.

When the vehicle passes immediately under the transmitting and receivingstation B, the skipping of the frequency does not theoretically occur inthe case of FIG. 9, whereas the skipping occurs in the case of FIG. 8.The reason for the occurrence of skipping in the case of FIG. 8 can beexplained as follows. Since two antennas, each having a differentdirection, are provided in a single transmitting and receiving stationin the present invention, the received radio waves from these antennasare exchanged when the vehicle passes immediately under the transmittingand receiving station B, thereby causing the skipping of the frequencycorresponding to double the amount of the deviation of the offsetfrequency.

Although the embodiment of the present invention has been describedabove, the present invention is not limited thereto. Various changes andmodification in the design can be made without departing from the scopeof the present invention.

What is claimed is:
 1. A road transmission equipment used for acommunication system between a road and a vehicle, which communicatesbetween a road communication station arranged in a cell and avehicle-mounted mobile station within the cell, comprising: a firsttransmitting antenna having a directivity in a running direction of avehicle, a second transmitting antenna having a directivity in theopposite direction to the running direction of the vehicle, a firsttransmitting section and a second transmitting section respectivelyconnected to the first transmitting antenna and the second transmittingantenna to output signals of the same frequency, and a frequencycorrection section, wherein; the frequency correction section providesthe first transmitting section with a positive frequency offset forincreasing the frequency of a signal supplied to the first transmittingantenna, and provides the second transmitting section with a negativefrequency offset for lowering the frequency of a signal supplied to thesecond transmitting antenna.
 2. A road transmission equipment as setforth in claim 1, wherein the amounts of the positive frequency offsetand the negative frequency offset provided by the frequency correctionsection are equal to each other.
 3. A road transmission equipment as setforth in claim 1, further comprising a speed detection means fordetecting the speed of the vehicle running in the cell, wherein thefrequency correction section sets the amount of the frequency offsetbased on the detected speed of the vehicle.
 4. A road transmissionequipment as set forth in claim 1, wherein the amount of the frequencyoffset provided by the frequency correction section is set to a fixedvalue on the assumption that the vehicle is subjected to constantDoppler shift.
 5. A road transmission equipment according to any ofclaims 1 to 4, wherein each of the first transmitting section and thesecond transmitting section transmits an orthogonal frequency divisionmultiplex (OFDM) modulated radio wave.